Polymethine dyestuffs



Patented Dec. 17, 1946 POLYMETHINE DYESTUFFS John Ilford, England, assign l'lford, England, a Britis No Drawing.

Serial No. 514,668. 1, 1943 Applica David Kendall and Harry Derek Edwards, ors to Ilford Limited, h company tion December 1'7, '1943,

In Great Britain January 11 Claims. (Cl. 260-240) This invention relates to polymethine dyestuffs and particularly to the production of dicarbocyanine and similar dyestuifs which serve as optical sensitisers for silver halide photographic emulsions.

In co-pending application serial No. 514,669,

where R1 is an alkyl group, R2 is a hydrogen atom or an alkyl or aralkyl grou and R3 is an alkyl or aralkyl group (including unsaturated groups of this type such as the allyl group and substituted groups of this type such as hydroxyalkyl groups), A is an acid radicle, D1 is the residue of a heterccyclic nucleus, and n is an integer less than 3. In general it is referred that the groups R1, R2 (if not hydrogen) and R3 should be lower alkyl groups, e. g. methyl and ethyl, and for convenience in preparing the compounds the group R1 is preferably ethyl. The group A in the foregoing formula represents an acid radicle, e. g. chloride, bromide, iodide, alkyl sulphate, p-toluene sulphonate and perchlorate.

In our copending application Serial No. 514,666, filed December 17, 1943, corresponding to British application No. 14/43, there are described the production of compounds of the general Formula A:

XR, R1s-c'H=cHoH XRI where the groups R1 are alkyl groups and the groups X are both oxygen atoms or both sulphur atoms.

Compounds of the type wherein X is S can be prepared as disclosed in aforesaid applications by treating with an alkyl mercaptan, in the presence of an acid condensing agent, a compound of one of the Formulae B, C, D, or E:

SR GHECH-CI/I (B) R CH-ECH-Cfl c) OR ao-0n=on-c (D) on RscH=cH-o (E) 01' by treating with an alkyl mercaptan in the presence of a basic condensing agent under pressure, a compound of Formula B.

Compounds of the general Formula B may be obtained by brominating acrolein and treating the product with an alkyl mercaptan followed by heating with an alkali to remove bromine. Compounds of the general Formula C may be converted to compounds of general Formula E by treating them with a sodium alkyl mercaptide and an alkyl mercaptan under pressure.

The most convenient compound of general Formula A to prepare is that in which the groups R are ethyl groups since ethyl mercaptan is the most convenient of the alkyl mercaptans to employ. In using the compounds of general Formula A as dyestufi intermediates the condensation to produce dyestuffs may eliminate all the SR groups so that the nature of the R group is not then of importance.

The acid condensing agent maybe any strongly acidic substance; sulphuric acid and zinc chloride have been found very suitable.

According to the present invention, a compound of the general Formula A is converted to a valuable dyestuff intermediate by condensing it in acid conditions with an alkyl, substituted alkyl, aralkyl or substituted aralkyl quaternary salt of a heterocyclic nitrogen base containing a reactive methylene group (including a methyl or mono-substituted-methyl group) in the alpha or gamma position to the heterocyclic quaternary nitrogen atom.

The condensation is preferably effected by heating the reactants together in the presence of a solvent for the reactants and in the presence of a compound capable of reacting with mercaptan or alcohol split off during the reaction. It is found that acetic anhydride is particularly suitable in serving both purposes.

The course of the reaction is believed to be as follows:

; oxazoles, oxazolines, selenazoles, selenazolines,

pyridine, quinoline, lepidine, indolenine, diazlnes (e. g. pyrimidine and the diazines described in 4 British patent specification No. 425,609), thioor an alkyl, aryl or aralkyl group. Examples are diazoles and q'uinazoline and the corresponding rhodanio acid (2-thio-4-lreto-tetrahydro thiasubstituted and unsubstituted polycyclic com zole),oxarhodanic acid -thio-i-keto-tetrahydro pounds such as the benzthiazoles, naphthathiaoxazole), and the N-alkyl, N-aryl and N-aralkyl zoles and anthrathiazoles and the corresponding 5 derivatives of such compounds, oxindoles, pyrapolycyclic oXazoles and selenazole's. Suchhetero- 'zole 5-ones. hydantoin, thiohydantoin, ip-hydancyclic compounds may contain 'substitiient groups toinand gl/'thio-hydantoin. in the benzene nuclei, e. g. alkyl, aryl, amino, hy- '(c) Carbo'cycli'c compounds containing a ketcdroXy and alkoxy groups or halogen'atoms. V methylene group, e. g. 1.3-indane-dione and the According to the present invention polymethin'e hydrindones. dyestuffs are obtained by condensing a compound ((1) Compounds of the general type (b) above, of general Formula I with a compound containbut which contain a substituent ing a reactive methylene group or a reactive R amino group. More specifically the invention is concerned with compounds which contain inethylene groups which are external to a ring system I v and rendered reactive by the character of the grouping (B. being hydrogen or a alkyl, aryl methyl and mono-substituted methyl which may 2,213,986. be written as -CH2R where R is a hydrogen atom (e) Hydrazines and substituted hydraz'ine's.

amino group in the or or position to the ouate'rc'yanine dyes. The reaction with a compound of nary nitrogen atom. Examples of compounds of class (a) is typified by the following general this type are those of the character defined in Equation E1:

which the heterocyclic nucleus is any or those Where the compound of class (a) contains a listed above in connection with the general Forreactive NHz group instead of the CHzR group mula I. The acid ad cle 0f the quaternary S t the product contains an =N- atominstead of the may be any of those listed above in connection with the group A in Formula I. g R

(b) Five-membered ring compounds of the genl eral Formula II:

f l group shown above. 'O o-orn (II) The reaction with a compound of class (D) is R typified by the following general Equation E2:

"'i 1 Io o-m R1s-oH=cH-on=o c=(c1rc11),.-1 1 I R Ra A :--D--: I'M '-"-Di-"--"-: F T-o0-o=oH-c11=oH-o=o-(o'H-o11 "4:1? ruse HA (E2) R Ra where D is the residue of a heterocyclic nitrogen The reaction with a compound of class (0) is ketomethylene ring and R is a hydrogen atom typified by the following general Equation E3:

F" "l i'""" (30-bit, R1s-o11=oH-o11=o o=(on-o11 m n r fi -'1 R: r'' --D1 ring y including such external groups as 'or aralkyl group) described in U. S. P. No.

" r an y yl r aralkyl group, and also meth- When employing com ounds of type (a) the vlene group which form part of a rin syst m products obtained are dicarhccyanine dyes. By and are rendered reactive by'theparticular 'enarusing a compound of type (a) in which the hetacter of the ring system. Specific general classes erocyclic nucleus is the same as that present in f mp fi fi g such reactive methylene 25 the intermediate of general Formula I, symmetand amino groups are; rica} dicarbocyanine dyes may be obtained, but (a) Quaternary salts of heterocyclic nitrogen the present invention presents the important adcompounds containing a substituent methylene, vantage that it affords a simple and practical 7 methyl or mono-substituted methyl group or an method of obtaining the unsymmetrical dicarbO- 'accordance with Equation E5.

In the above formulae and equations R, R1, R2, R3, D1, A, and X have the same values given On coolin the dye crystallised and was filtered off, and'washed well with ethyl alcohol, water in Formula I and D2 has the same values as D1. and ether. It had a melting point of 263C. with When employing compounds of type (b) such decomposition. This dyestufi when incorporated as rhodanic acid Or oxarhodanic acid or th corin a gelatino silver iodobromide emulsion imparts responding N-substituted compounds, the resulta 'band of sensitivity with a, maximum at 6900A. ing product contains a thio group and can be r further reacted in a manner analogous to that EXAMPLE m 1 described in U, s. application N 136,251, r-- Preparation of Z-methyl 1'-ethyl thia(2')quino responding to B. P. No. 489,335. dicarbocyanine iodide The condensation may conveniently be eflected L i n by heating the reagents together in the presence gg g gg z ggg i 2 21 33 55 of a base, for example an Organic base h as ethiodide were boiled gently under reflux until pyridine or triethwaminefir an inqrganic base mostly dissolved in .150 cos. of ethyl alcohol. 2.1 Such as an M alum W cc's. of triethylamine were added andtheheath followmg examples Illustrate the mven ing continued for 12 minutes. The dye crystaltlo he p s f p e I to V VII 3 1 lised from thesolution and after cooling was fil- XV proceeds m accordance tered off. Recrystallisation from methyl alcohol 3 0 332 3 5k g gqgi gfi g g g g gg i 2 frlielded the dyezag green igygtals withta gold re- Y ex melting at C. Wi ecomposi ion. This XII p g a e E1 as P 5 dyestuif when incorporated in a gelatino silver treatises irifiiiilifitltriiiii' tit emu-eon W a e Equation E3; that of Example XV in accordance ,tlvlty 2 F m -in' 715p and e with Equation E4 and that of Example xvn in mg 7 y EXAMPLE IV EXAMPLE I l Preparation of Z-methyl1':3':3'-trimethyl mmd we r Preparationof Z-methyZ-P-p-hydromyethyl thiam 0 z a bocya "f wdz t (4') quino dicarbocyanine iodide th -9 1 -t t g 3 iazoe me 1013 31). gmso rfl hlO: hllh. C. droxy ethiodide were warmed into solution in f gg z g f g igg g 5 P ethyl alcohol of triethylamine were tate was added and the heating continued for 12 addedand the Whole boned gently under reflux minutes. on allowing to cool, the dye crystal- 23ft tttiiiiiithtiliinitiit2.5532 1122252235; 533 3315 3; 5 1 213;} gggg ggg: mittltriittflififtitii ttf fiiififi 33311; gg ggfiggggfgggg ggeggi gzg hol, water ethyl alcohol again, and finally f with decomposition. 'This dyestufi when incor fifi'fiiiltfiitit. 5,232.22:Zt12f fi?tt t% tutti; :tttztz.ittttstis zmitt: with decomposition. This dyestuff when incorat 6850 A and extendinplt'o 7400 A porated in a gelatino silver iodo-bromide emulsion v imparts a band of sensitivity with a maximum at i EXA P E V I moo A. Preparation of z ethyz-r memyr omkzoqum- .7 II 1 f dicizrbocyanme iodide Pemmtm of 'gme ig wwe 10. cos. of. a. solution ofI-Q ethyl-thiQ 'butaQ 7 cyan M w z e dienyl benzoxazole ,ethyl-p-toluene sulphonate 1.93 gins. of l-w-ethyl-thio butadienyl *benz- (prepared as in Example IV of (to-pending Applithiazoleimethiodide...andt2.'75--gms. .of.,1-methy1 :7'5 .cation- Serial No. 514,669 corresponding to Brit- 10 cos. of a solution of l-w-ethyl-thio butadienyl 'benzoxazole ethyl-p -toluene sulphonate (prepared as inExample. IV'of co-pending application Serial No. 514,669 corresponding to British specification No. 15/43 and 0.37 gm. of N- methyl r-hodanic acid were warmed together on the water bath. 0.4 cc. triethyl'amine was added and. the whole refluxed for 10 minutes. The dye was. filtered off, after cooling. the reaction mixture, recrystallised from methyl alcohol in which it is. difli-cultly soluble, and thus obtained as blue needles. M. pt. 215 C. This dyestuff when incorporated in a gelatino silveriodobromide emulsion imparts a band of sensitivity with a maximum of 6500 A. and extending to 7000 A.

. EXAMPLE? VII Preparation of z eth'yl 1:3:3'-trz'methyl .71amdodicarbocyanine perchlorate ccs. of asolution of e-ethyl-thio butadienyl 'benzoxazole ethyl-p-toluene sulphonate (prepared: as in Example IV of co-pending application Serial No. 514,669 corresponding to British specification No. /43) and 0.75 gm. of 2:3:3- trimethyl indolenine methiodide were warmed together 'on the water bath. 0.7 cc. of triethy-lamine was added and. the; reaction mixture boiled gently under reflux for 12. minutes. The dye was precipitated on cooling by dilution with water, alcohol and ether, and had a M. pt. of 158 C. with decomposition. It'was converted to the perchlorate saltby dissolving: it in IOmcs. ofmethyl alcohol and-filtering the hotsolution into a hot aqueous solutionof potassium perchlorate. The dye crystallised outand wasfiltered off. After crystallisation from methyl. alcohol solution, it was obtained as violetcrystals. M. pt. 140 C. with decomposition This dyestufi whenincorporated in agelatino silver iodo-bromide emulsion imparts a band. of sensitivity with a maximum at.6400 A.

EXAMPLE VIII .816- gins of lw-ethyl thiobutadienyl. benz- M. pt. 175 0. with decrystals; M. pt. 221

Preparation of 2-rnethyl-2'-ethyl thiaselenodicarbocyanine iodide --6 gins. of w-ethyl-thio butadienyl benzselenazole ethiodide and 4.8 gms. l-methyl benzthiazole methiodide were warmed,.till mainly in solution, with IOOccs. alcohol. 4.6 cos; of triethylamine were added and the reaction mixture boiled gently under reflux for ten minutes. The dye-began to crystallise from the-solution, which was diluted with a little water and cooled. The dye was filtered oil and thus obtained as green crystals. M. pt. 216 C. With decomposition. This dyestuff when incorporated in a gelatino silver iod-o-bromide emulsion imparts a band of sensitivity with a maximum at 7050 A and extending to 7400 A.

EXAMPLE X Preparation of Z-ethgl 1-3-3-trimeth;yl selerioindodicarbocyanine perchlorate gms. of w-ethyl-thio butadienyl benzselenazole ethiodide, 4.6 gms. of 213-23-trimethy1. in-

dolenine methiodide and 100 ads. of ethyl alcohol were warmed together and 4- cos. ortriethylami'ne then added. After boiling gently under'reiiux thiazole methiodide and 7.1 gins. of l-methyl benzthiazoleafl hydroxy 'ethiodiole were boiled gently together under-refluxin 150 cos. of ethyl alcohols. 7 cos. of. triethylaminewas added to the hot solution and the heating. continued for 12 minutes. At this stage the dye began to crystal- *lis'e from the solution. Therea'ction mixture was -'poure'd:into a little water, cooledandth'e dyefiltered off'; A'fter recrystallisation" from methyl alcohol the dye wasobtained aszshininggreen for 10 minutes the hot solution was poured-- into a warm aqueous solution of potassium perchlorate.- The liquors were clarified with acetone; and the dye crystallised from the solution. Recrystallisatiorr of the dyestuff from methyl alcohol gave dark blue crystals. M. pt. 197 C. withdecomposition. A second crop from the: recrystallisation had M. pt.- 204" C. with decomposition. This dyestufi when incorporated in a gelatino silver iodo-bromide emulsion imparts a band of sensitivity with a-maximum at 6875 A and extending to 7000 A.

V EXAMPLE XI 7 Preparation 0 1! '3 -methyl 1. :3 :3"-trime'ihyl'f mazoZi'no-indo dicarbocyanin'e. perchlorate 2 gms. of ,a-methyl thia'zoline and 3.5" g'ms. of methyl-p-toluene sulphonate were fused together at C. for one hour. The quaternary salt was then warmed, partly into solution, with 7 gins." of 2-w-ethyl thio butadienyl lt3't3-trimethyl' indolenium perchlorate and-50 cos. of ethyl alcohol. 6: cos of triethylamine was added andthe reacti'on mixture boiled gently under refluxfor 1-0 minutes. The dye bumped o'ut of' solution; The reaction mixture was diluted with ethyl-ea-lcohol and water; and cooled. By filtrationthe'dye was obtained: as bright green crystals. M. pt. 227 C. with decomposition. This dyestufi when incorporated in a gelatinosilver io'do-bromideemulsion imparts a band of sensitivity'with a maximum at 6200 A. Used-in larger quantity'it gives amaximum sensitivity at 6850 A.

EXAMPLE Preparation 0 2E2-dimet7iyl17-az tiiin time cyam'ne iodide I 0.3 gm. of, l-amino. benzthiazole and l gm. of methyl p-toluene sulphonatewere' fused together at 100 C. for /2 hour. The solid quaternary salt thus formed: was dissolved: in- 60 ccs.- of: pyridine and 1.6 gm. of l-w-ethyhthioibutadienyL-benz alcohol was EXAMPLE XIII Preparation of '[1-methyl-dihyctroquinolylidene- 2] -butenyZidene-1 3 -dilceto-hydrindene 0.18 gm. of 2-(w-ethyl-thio-butadienyl) -quinoline methyl perchlorate and 0.08 gm. of 1:3 indandione were boiled gently under reflux with 7 cos. of a 2% ethyl alcoholic solution of sodium acetate for one hour. The reaction mixture was cooled and diluted with a little water. The precipitated dycstufi was filtered off and washed with ethyl alcohol, water and ethyl alcohol again. The dye was purified by boiling out with methyl alcohol and filtering hot, when the sparingly soluble dyestuff remained as brown crystals. M. pt. 268 C. with decomposition.

EXAMPLE XIV Preparation of Z-methyZ-I-ethyZ-'.6'-benzthia(4')quino dicarbocyanz'ne iodide 0.19 gm. 1-(w-ethyl-thio-butadienyl) -benz thiazole methiodide, 0.18 gm.of 5.6- benz lepidine ethiodide and cos. of ethyl alcohol were boiled gently into solution under reflux. 0.14. cc. of triethylamine was added and the heating continued for 3 minutes. The reaction mixture was then allowed to stand overnight, when the dyestufl? crystallised out. It was filtered off and washed with water, ethyl alcohol and ether. Recrystallisation from methyl alcohol yielded the dye as green crystals. M. pt. 180 C. with decomposition.

EXAMPLE XV Preparation of 7- [1 emethyl l .Z-dihydroquinolyiz'dene 2] 2 [I phenz/Z 3 methZIl-5-keto- 4.S-dihi /dropt/raeolylidene-4l-3.5-heptadiene EXAMPLE XVI Preparation of Z-o-acetoryethyl-Z-p-hydroa:y-

ethyl-thiadicarbocyanine iodide 1.6 gm. of 1-methyl benzthiazole e-hydroxyethyl iodide and 25 cos. of acetic anhydride were boiled gently under reflux until the solid passed into solution. 2 cos. of fl-ethyl-thio-acroleinacetal were added and the reaction mixture heated for a further 10 minutes. The excess acetic anhydride was then distilledoff under reduced pressure, and the residue was washed with petroleum ether and ether. This intermediate was further reacted with 1.6 gm. of 1-methy1 benzthiazole-c-hydroxyethyl iodide by heating with 20 cos. of ethyl alcohol and 1.4 cc. of triethylamine for 5 minutes. 'More added and the dye solution was alkyl and aralkyl groups, R:

10 filtered hot from a little tarry residue, and poured into anaqueous solution of potassium iodide. The dye precipitated and was filtered off. After recrystallisation from methyl alcohol it was obtained as crystals. M. pt. 210 C. with decomposition. This dyestulf, when incorporated in a silver iodo-bromide emulsion, imparts a band of sensitivity with a maximum at about 70005.

EXAMPLE XVII Preparation 0; the dz'hydriodide of 6-(1-methyldihydrobenzthiaeolyli ene) -crotonaldazine 0.13 gm. of powdered hydrazine hydrochloride was boiled gently under reflux with 40 cos. of a 2% solution of sodium acetate in ethyl alcohol. 1.0 gm. of 1-(w-ethyl-thio-butadienyl) -benzthiazole ethiodide was added and the heating continued for 30 minutes. A red colour rapidly appeared, and at the end of the heating period the dye solution was filtered hot. On dilution of the filtrate the dye was obtained as dark purple crystals, M. pt. 204 C. with decomposition. Recrystallisation from methyl alcohol yielded the dye as dark brown crystals, M. pt. 208 C. with decomposition.

What we claim is:

1. Process for the production of polymethine dyestuffs which comprises condensing a compound of the general formula:

where R1 is an alkyl group, R2 is selected from the class consisting of the hydrogen atom and alkyl and aralkyl groups, R: is selected from the class consisting of alkyl, aralkyl, allyl and hydroxyalkyl, D is the residue of a heterocyclic nitrogen nucleus usual in the cyanine .dye art, Ais an acid radicle and n is an integer less than three, with a compound containing a reactive group selected from the class consisting of reactive methylene groups and reactive amino groups.

2. Process for the production of polymethine dyestuffs which comprises condensing av compound of the general formula:

2 :-D----: R.s-cH=oHoH=c-o= on-omm n where R1 is an alkyl group,

R2 is selected from the class consisting of the hydrogen atom and is selected from the class consisting of alkyl, aralkyl, allyl and hydroxyalkyl, D is the residue of a heterocyclic nitrogen nucleus usual in the cyanine dye art, A is an acid radicle and n is an integer less than three, with a cyclic compound containing a methylene group rendered reactive by the ring system.

r 3. Process for the production of polymethine dyestufis which comprises condensing a compound of the general formula: 1

the class consisting of the hydrogen atom and alkyl and aralkyl of alkyl, aralkyl, allyl and Rls-cH;oH-cn=.o-o; on-o11 1=N Ra NA where R is an alkyl group, R2 is selected from the class consisting of the hydrogen atom and a lkyl and aralkyl groups, is selected from the class consisting oi a lhyl, aralkyl allyl and bydro zryalkyl, D1 is theresidue of a heterocyclic nitrogen nucleus usual in the cyanine dye art A is eh' is eel el a h e an integer l ss t an three, w h? eemh eh l 9 t e g neral ier u w x i-o o-on, R

wherein D2 is the residue of a heterocyclic nitrogen ketomethylene ring and R. is selected from the class consisting of the hydrogen atom and hydrocarbon groups.

5. Process for the production of polymethine dyestufis which comprises condensing, in the Pre n o e base e em ouh f t ener formula:

where R1 is an alkyl group, R-z is selected from the class consisting of the hydrogen atom and alkyl and aralkyl groups, R3 is selected from the class consisting of alkyl, aralkyl allyl and bydroxyalkyl, D is the residue of a heterocyclic m't-. rogen nucleus usual in the cyanine dye art, A is an acid radicle and n is an integer less than three, with a cyclic compound containing a methylene group rendered reactive by the ring system.

6. Process for the production of polymethine dyestuffs which comprises condensing, in the presence of a base, a compound of the general formula:

Wh re R1 i a lk 8.1 11 R215 se eted 10m he-class c si tin o the h d n om and alkyl and aralkyl groups, R; is selected from the class consisting of; alkyl, aralkyl allyl and hydroxyalkyl, D is the residue of a heterocyclic nit o h n cl us sual t am dye art. A is an acid radicle and n is an integer less than three, with a quaternary salt of a heterocyclic nitrogen compound containing a substituent group selected from the class consisting of methylene and amino groups in one of the cc and positions to the heterocyclic quaternary nitrogen atom.

Ereeeee r he Q he ien of n lym thih dyeetufis w ch om r e condensi in th pr eehee o a. b se, a. eq h enhd Qt. he en ra fo ula: r I

'- zines.

elasece sis ine; of me h ene where R1 is an alkyl group, He is selected from the class consisting of the hydrogen atom and alkyl and aralkyl groups, R3 is selected from the class consisting of alkyl, aralkyl allyl and hydroxyalhyl Di is the residue of a heterocyclic nitrogen nucleus usual in the cyanine dye art, A isan acid radicle and nis an integer less than three, h a e peuhd o the gene o m la:

i wherein D2 is the residue of a heterocyclic nitrogen ketomethylene. ring and R is selected from th e s ee ihe e he y og n a m and hyd a b g TQQ S:

8. Process for the production of polymethine dyestuffs which comprises cond n ng a 6,011.1- pound of the general formula:

where R1 is an alkyl group, R2 is selected from the class consisting of the hydrogen atom and alkyl and aralkyl groups, R; is selected from the class consisting of allryl, aralkyl 'allyl and hydroxyalkyl, D is the residue of a heterocyclic nitrogen nucleus usual in the cyanine dye art, A is an acid radicle and n is an integer less than three, with a compound selected from the class consisting of hydrazines and substituted hydra- 9. Process for the preduetion of polymethine dyestuffs which comprises condensing a comp und of the e r thrmula:

a cyclic compound containing a methylene group rendered reactive by the ring system.

10. Process for the production of polymethine dyestuffs which comprises condensing a compound of the generalfgrrnula;

T'Ra. A

Where e g o ps R1. e dRi er o r allsy g eens and t ro t? B2 i e r up of the st uctur --(ICH2 )-1.-.H where .ris an integer less than i, D e t e e ue of e h t r eyel'ie itr gen nucleus us al; he eyehine ye A is an acid; vesic e an n s nt e ess han hree.

t sal e a tereer lie h tree, 1 i e euh titheht. em Se t emi one oi the send 1 os ti ns to. t ehete eerel e uat nary" n t o n etch. Pr ss r t e p o uct on, of p ymethine 13 dyestufis which comprises condensing a compound of the general formula:

nls-on=on-on=e-o=(on-cn M n R: A where the groups R1 and R3 are lower alkyl groups and the group R2 is a group of the structure (CH2)a:-1H1 where :1: is an integer less than 4, D1 is the'residue of a. heterocyclic nitrogen nucleus usual in the cyanine dye art, A is an acid radicle and n is an integer less than three, with a. compound of the general formula:

JOHN DAVID KENDALL. HARRY DEREK EDWARDS. 

